#Square brackets ([ and ]) delimits a range of characters.!

[aA] means either a or A. 
[a-z] matches any lowercase character.
[0-9] matches any digit. 
[0-9a-zA-Z] for characters commonly used in variable names.

You can combine the brackets with other patterns.

Pattern         Interpretation
/[aA]/          Matches against a or A.
/[aA]+/         Matches one or more instances of a or A.
/[aA]*/         Matches zero or more instances of a or A.
/[aA]?/         Matches zero or one instance of a or A.
/[^aA]/         Returns true if any character is found that is not a or A.
/[aA]|[bB]/     Matches an instance of a or A or b or B; redundant in this case, as it is the same as /[aAbB]/.

   
    
    
#The [] special characters enable you to define patterns that match one of a group of alternatives.!

For example, the following pattern matches def or dEf:

/d[eE]f/

   
    
    
#Combine [] with + to match a sequence of characters of any length.!

/d[eE]+f/

This matches all of the following:

def
dEf
deef
dEef
dEEEeeeEef




#The bracketed character class!
    
while(<DATA>){
   print if /[A-Za-z0-9_]/;
}
__DATA__
    Tom 101
    Jack 201
    Nart 301